Lotta factory rifles to choose from in 54, and it's certainly easier to find components if you're not making your own.
Decide on a 58 and you'll be looking for a used gun or having one built, and you wont find components on just any shelf in any store.
On the other hand, if you already own a 50, the jump to 54 just isn't going to seem like a big switch.... Or at least that's the way it is for me. Skip a caliber, as in jumping from 50 to 58, and you'll definitely feel the difference.
As for deer/black bear, heck even elk and maybe moose, the 54 is dandy. Another having-said-that however, since I got sidetracked with 58's my 54's don't get much field time. Not a reflection on the 54 at all, rather the 58 is just a lot more entertaining to me.
Oh, what the heck.... How about throwing in a on-the-other-hand to go with my having-said-that. :grin:
Slip a heavy conical down the bore of either a 54 or a 58 and put enough powder behind it to flatten the trajectory for 100 yard shots, and you'll get your eyes opened. And become a serious fan of round balls! :rotf: